Transconductance signal capacity format

ABSTRACT

Operational transconductance amplifiers have a natural signal capacity format in which signal performance can be expressed in terms of fixed percentages. Input signal can be applied to Operational transconductance amplifiers in this natural signal capacity format in order to optimize performance. A signal which drives a given Operational transconductance amplifier architecture to produce an output current which is at 50% of it&#39;s maximum available output current can be thought of as applying an input voltage which is at 50% of an Operational transconductance amplifier&#39;s maximum input voltage capacity. In this input/output channel capacity format, dc offset, distortion, and noise all are temperature independent. By translating input signal between a voltage format to a channel capacity format using the methods of this invention, output signal performance attributes such as gain, frequency response, dc offset, temperature drift, distortion, and noise, can all be optimize over the full temperature range for all types of Operational transconductance amplifier architectures and applications.

FIELD OF THE INVENTION

This invention relates to processing signal in OperationalTransconductance Amplifiers in terms of a native channel capacity formatinstead of in terms of voltages or currents. In the channel capacityformat, all performance attributes become stable over temperature. Sincethe level of input signal is chosen to be a compromise between all theperformance requirements, an optimum level chosen at room temperaturewill remain optimum over temperature. The transformation of input signalfrom voltage or current format to and from a channel capacity formatrequires some extra circuitry and magnitude adjustments. The intentionof the invention is to be to able to operate a multiple of seriallyconnected Operational Transconductance Amplifier stages such that theoutput signal is maintained at an optimum level of performance in termsof gain, frequency response, dc offset, temperature drift, distortion,and noise, in a manner independent of temperature.

BACKGROUND

The signal level chosen to apply to Operational TransconductanceAmplifier is a compromise between performance requirements. Noise, andDC offset encourage larger input signal voltages. Distortion encourageslower input voltage signals. In Operational Transconductance Amplifiers,noise, DC offset, distortion, and gain are all varying as a function oftemperature to the extent that performance is often degraded. NewerOperational Transconductance Amplifiers employ some distortioncancellation techniques to allow larger levels of input signal voltage.The input stage of FIG. 1 was proposed by Okanobu in U.S. Pat. No.4,965,528 issued Oct. 23, 1990. FIG. 1 shows this input stage usestransistor area scaling to apply equal and opposite DC offset voltagesto two simple differential input stages. Both input stages are biased upwith equal currents It1 a and It1 b. At the right magnitude of N, thedistortion from transistors Qn1 a and Qn1 d are cancelled by the equaland opposite distortion from Qn1 b and Qn1 c. FIG. 1 adds a PNPs Qp1 aand Qp1 b to subtract the difference between the collector currents toproduce a differential output current. The distortion of such anOperational Transconductance Amplifier architecture using a N value a 5is shown in FIG. 2. For input signal expressed in terms of voltage, FIG.2 shows that different levels of peak input voltage will have differentlevels of distortion at different temperatures. The distortion curvesscale to absolute temperature. All Operational TransconductanceAmplifiers without resistors, have this same relationship totemperature. This also applies to CMOS input stages which operate in thesubthresshold region.

In addition to distortion, Operational Transconductance Amplifier seetemperature variations in DC offset, noise, and gain. Thetransconductance transfer curve for the circuit of FIG. 1 is shown inFIG. 3. Gain is also has a direct relationship with absolutetemperature. In most Operational Transconductance Amplifierapplications, it is desirable for the voltage to current relationship ofthe transfer curve to come close to matching a perfect resistor. In gainor filter applications, an Operational Transconductance Amplifier stagewill be functioning like an electrically controlled resistor. Thisinvention teaches that the very steps that are needed to make anOperational Transconductance Amplifier behave like a perfect resistoralso naturally set up the operation in the channel capacity format. Theinvention shows how to operate Operational Transconductance Amplifiersin a way that holds both impedances and performances to be constantregardless of temperature.

BRIEF SUMMARY OF THE INVENTION

This invention translates signal for a given OperationalTransconductance Amplifier architecture into a channel capacity formatfeaturing of both constant impedances, and constant performance,regardless of temperature. The intention is to be to able to operate amultiple of connected Operational Transconductance Amplifier stages suchthat the output signal maintains an optimum level of performance interms of gain, frequency response, dc offset, temperature drift,distortion, and noise, in a manner independent of temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments are described with referenceto the following drawings:

FIG. 1 illustrates an Operational Transconductance Amplifier stagesusing a distortion cancellation architecture to extent input voltagerange and lower distortion. This differential input stage was proposedby Okanobu in U.S. Pat. No. 4,965,528 issued Oct. 23, 1990.

FIG. 2 illustrates the distortion of the Operational TransconductanceAmplifier stage shown in FIG. 1. The distortion is shown to be variablein terms of the peak input voltage, and the peak value relative totemperature.

FIG. 3 illustrates the transfer function for the OperationalTransconductance Amplifier stage shown in FIG. 1A, as a function oftemperature.

FIG. 4 illustrates a standard Operational Transconductance Amplifierstages consisting of a two transistor differential input stage andconnected to a turnaround.

FIG.5 illustrates the distortion of the Operational TransconductanceAmplifier stage shown in FIG. 1, but plotted in terms of channelcapacity, in which distortion is in a temperature independent format.

FIG. 6 illustrates the transfer function for the OperationalTransconductance Amplifier stage shown in FIG. 1 in the channel capacityformat, in which the transfer curve has become dimensionless andtemperature independent.

FIG. 7 illustrates that biasing a Operational Transconductance Amplifierwith a current scaling to absolute temperature allows Transconductanceto become temperature independent.

FIG. 8 illustrates how biasing a Operational Transconductance Amplifierwith a current scaling to absolute temperature also generates voltagesignals compatible with the channel capacity signal format.

FIG. 9 illustrates how biasing a Operational Transconductance Amplifierwith a current scaling to absolute temperature also loads the output ofan Operational Transconductance Amplifier with current signalscompatible with the channel capacity signal format.

FIG. 10 illustrates a example 4 pole lowpass filter application which isusing the channel capacity signal format.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A simple representation of an Operational Transconductance amplifier isshown in FIG. 4. The transconductance action is taking place intransistors Qn2 a and Qn2 b which convert a differential input voltageapplied to the two bases into a differential output current between thetwo collectors. Transistor Qp2 a and Qp2 b are performing the currentsubtraction process of a current turnaround such that a differentialoutput current appears at the output node. The output performance ofthis Operational Transconductance amplifier is specified in terms ofgain, offset, distortion, and signal to noise error. These errors can bemodeled as an array of small voltage sources connected in series withthe input signal. All of these referred to input error signals arechanging with temperature.

The fact that the DC offset voltage will vary with temperature meansthat an Operational Transconductance amplifier can operate like a realtime temperature sensor. In applications where the electronics drawenough power to require ventilation, any fluctuations in temperaturewill appear as a low frequency background noise signal. A typical DCoffset of 1 mV will vary by 3.3 uvolts for every degree change inambient temperature. The low frequency 1/f noise in bipolar transistorshas a peak-to-peak value about six times less.

An Operational Transconductance amplifier can be defined to be operatingat 50% of its capacity when it is producing an output current that is50% of of its maximum output current capacity. When operating at 50% ofits output current capacity, it can thought of receiving an inputvoltage at 50% level of what is essentially it's maximum input voltagecapacity. The signal in the channel capacity format can be treated asbeing as being at the same percentage in terms of either input voltagecapacity or output current capacity.

An input DC offset for bipolar transistors is modeled as a mismatch intransistor area. A 1 mV voltage offset corresponds to about a 4% areamismatch between transistors Qn4 a and Qn4 b in FIG.4. The equation (1)that it follows is below.

(Vbe2−Vbe1)=(kt/q)*ln(Area1/Area2)   (1)

In FIG. 4 if Qn4 a is 4% larger than Qn4 b, then collector current Ic4 awill be 2% higher that one half the current of It4 a, and the collectorcurrent Ic4 b will be 2% lower that one half the current of It4 a graph.Since the maximum available output current for FIG. 4 is It4 a, therewill be a DC offset current at the output which will remain at aconstant 2% of maximum available output current, regardless oftemperature. In the format of input voltage, DC offset for a 1 mV offsetappears to move 3.3 uV for every degree change. In the percentage formathowever, offset appears as a temperature independent 2% output currenterror.

The input voltage for the Operational Transconductance amplifierarchitecture of FIG. 4 is around 52 mV at room temperature. The verysame 1 mV of DC offset voltage corresponds to a 2% of maximumrecommended input voltage. But the maximum recommended input voltagescales directly with absolute temperature like the DC offset voltage. Soa 2% DC relative output current error also corresponds to a 2% DCrelative input voltage error regardless of temperature. When inputsignal is converted to a channel capacity percentage format, it willtherefore scale directly with absolute temperature. In doing so, it willremain the same magnitude relative to the DC offset, regardless oftemperature.

Different Operational Transconductance amplifier architectures will behaving different values for a maximum recommended input voltage. Thearchitecture of FIG. 1 has a level around 90 mV when the value N is setclose to 5. There are other Operational Transconductance amplifierarchitectures with twice that level. CMOS transistors operating in thesubthresshold region operate at about a 50% larger input voltagerelative to bipolars using the same architecture. All such architecturesoperate under a modified version of equation (1).

The most dominate noise in bipolar is shot noise. This noise is reallyindependent of how a bipolar transistor is fabricated. Shot noise isstatistical in that it is defined by how many electrons or holes flowacross an emitter base junction. Defined in terms of being an rmscurrent relative to the maximum available output current, the equation(2) is given below.

I _(—) rms/Idc=sqrt(2*q*Bandwidth/I _(—) dc)   (2)

The shot noise equation (2) shows that the channel's signal to noiseratio is purely a function of the number of charged particles flowingacross an emitter base region. Where as a the circuit of FIG. 4 maytypically have a DC output current error with a standard deviation of 2%of the output channel capacity, the rms value of output noise currentover a audio bandwidth is closer to 0.001% the output channel capacity.Noise can be modeled as moving offset. Signal converted to a channelcapacity percentage is also be temperature independent relative tobackground noise

The distortion curve of FIG. 2 shows the dependency of input distortionas a function of temperature for the circuit of FIG. 1. The curve ofFIG. 5 is the same curve translated to the channel percentage capacityformat. The temperature effects are removed, and so an OperationalTransconductance amplifier operating with signal at a fixed channelcapacity percentage level, should have distortion, dc offset, and noisebe independent of temperature.

Operational Transconductance amplifier are typically used within theirlinear region, as shown in FIG. 6. Because of the linearity within the50% region of available output current, the one to one relationshipbetween inputs and outputs in terms of percent values holds pretty well.A 2% error of offset can be modeled as either applying a 2% relativeinput voltage or a 2% relative output current.

The voltage to current relationship or transconductance is also afunction of temperature. FIG. 3 shows the transfer curves for thecircuit of FIG. 1 as a function of temperatures. If the bias current ofan Operational Transconductance amplifier is also set to scale toabsolute temperature, transconductance become temperature independent asshown in FIG. 7. At the low input levels, it is desirable that anOperational Transconductance amplifier can behave like a perfectresistor which does not drift with temperature. There is a type of onchip resistor, the SiChrome resistor, which operates close to having azero TC. This type of resistor has much tighter resistance tolerances.Such a resistor can be used together with a bandgap to generate such abias current for an Operational Transconductance amplifier. If thetransconductance is behaving like a perfect resistor, then connecting alike a perfect resistor to an Operational Transconductance amplifier'soutput will product a gain stage that is stable over temperature. Theapplication shown in FIG. 8 is that of an attenuation stage. The currentvalue of Ibias8 a is set to be maximum at 1 uA where the gain will beone. Gain is designed to decrease by reducing the value of Ibias8 a.

When the bias current of an Operational Transconductance amplifier isset to scale to absolute temperature, it provides an additional feature.If the Operational Transconductance amplifier architecture defines inputvoltage capacity to be 90 mV, then the values of Ibias8 a and R8 a canbe designed to provide the same output voltage capacity to also be at 90mV. If OTA8 a is being driven at 50% of its capacity, it will in turndrive OTA8 b at no higher that 50% of its channel capacity. The conceptof constant impedance at constant performance is being shown in FIG. 8.All signal voltages and currents in FIG. 8 will scale directly withabsolute temperature. Since both voltages and currents scale together,the transconductances will be independent of temperature. The signal isbeing held at a constant percentage level, so the performance can beheld to be temperature independent as well. Input signal is thereforeprocessed at constant impedance and performance by applying this nativepercentage format to Operational Transconductance Amplifiers.

The filter application shown in FIG. 9 illustrates how percentage formatalso works in terms of loading the outputs of OperationalTransconductance Amplifiers. In this filter application, the OperationalTransconductance Amplifiers with their output capacitors are operatingas integrator blocks. The feedback set up resistors R9 a, R9 b, and R9 cdefine the Q term of transfer function given above. The transconductanceand capacitors define the frequency response. At low frequencies, theoutput capacitors C9 a and C9 b are high impedance and therefore do notload OTA9 a and OTA9 b. When the frequencies are high enough to require10% of the available output current, this will be reflected as requiringa 10% of recommended input voltage being applied across the inputs ofOTA9 a and OTA9 b. As in FIG. 8, all signal voltages and signal currentswill scale to absolute temperature. But with the transconductance beingtemperature independent, the corner frequency of the filter will remaintemperature independent. The output capacitor will be swinging bothhigher voltages and currents at higher temperatures, but the percentageloading will remain the same at both the output and the input,regardless of temperature. So with signal in the percentage format, bothgain and frequency response can be optimize for performance overtemperature as well.

The invention's method to stabilizing impedances does have one drawback.When the bias currents to Operational Transconductance amplifier stagesare change, their relative noise changes as shown in equation (2). Overthe full temperature range, this amounts to about a ±1.3 dB change inthe signal to noise ratio. The prior Art shown in FIG. 4 has a ±2.45 dBchange in the signal to noise under the same conditions.

FIG. 10 shows a preferred embodiment of the invention. This embodimentis using a four pole low-pass filter in the Signal Processor 10 b as anexample of a multiple of interconnected Operational Transconductanceamplifier stages being designed to produce an output signal at optimumperformance. The Input Converter 10 a at the top left shows OTA10 abeing driven to with a percentage based voltage signal to produce apercentage base output current. This signal is coupled to all the otherOperational Transconductance amplifier stages.

Input signals are normally voltage signals with a limited magnitude. Thevalues of bias current Idc10 a and resistor RIN10 a are both chosen suchthat a maximum peak input voltage at RIN10 a will require 50% of themaximum available output current of OTA10 a. Operation Amplifier OpAmp10a is driving the input to OTA10 a to cancel the current coming in fromRIN10 a. If both the resistance to RIN10 a and the bias current Idc10 aare constant with temperature, then this 50% channel capacity for OTA10a will be constant with temperature too. The output to OpAmp10 a canthen be used to drive everything in the Signal Processor 10 b block.

In the Signal Processor 10 b, signal gets processed in each OperationalTransconductance amplifier stages at a percentage of capacity mode. Thebest way to monitor the performance of the Operational Transconductanceamplifier stages is to monitor the input voltage as a percent of theinput voltage capacity over the full frequency range. Once theperformance has been optimized at room temperature, it will remainoptimized over temperature.

The final stage involves the conversion from the channel capacity signalback into the standard voltage format. If the bias current Idc10 f andresistance of ROUT10 f were to both be temperature independent, then afixed percentage signal will produce a fixed output voltage.

The invention is intended to stabilize transconductance while optimizingperformance over temperature for an application using a multiple ofOperational Transconductance amplifier stages. The preferred embodimentshows the methods involved in the temperature scaling of both the signalvoltages and bias currents for all Operational Transconductanceamplifiers in the signal path, thereby stabilizing both impedances andperformances. The methods include a channel capacity format by which tomeasure and design the performance in each and every OperationalTransconductance amplifier stage. Included is the circuitry with itsmethod to translate input signal voltage in to channel capacity formatusing channel capacity format to optimize the actual signal conversion.The circuitry and methods to convert signal back into a voltage formatare also included.

While the invention has been shown in this particular embodiment, itwill be understood by those skilled in the art that the OperationalTransconductance Amplifier architectures are not limited to the oneshaving been shown. Since the applications are can be parallel or serial,and are not limited filter or attenuator applications. Likewise, CMOSdevices can be substituted for bipolar devices. All of thesesubstitutions can all be made with out departing from the spirit andscope of the invention

1. A method and architecture for converting an input voltage signal fora given Transconductance Amplifier Architecture into an outputpercentage capacity format comprising: a multiple of biasing currentseach having an individual magnitude with a common absolute temperaturescaling being used to current bias a multiple of TransconductanceAmplifiers such as to provide both a means of temperature stabilizingtransconductance while also providing a means of generation for amultiple of voltage signals at a multiple of circuit nodes each havingan optimum individual magnitude with the common absolute temperaturescaling; such that the input voltage signal can be coupled to themultiple of Transconductance Amplifiers in the output percentagecapacity format; such that an optimum output signal performance levelrelative to distortion, gain, transconductance, frequency response, DCoffset, temperature drift, and noise can be maintained over a widetemperature range for the given Transconductance Amplifier architecturefor a multiple of applications.
 2. A method and architecture forconverting an input voltage signal for a given TransconductanceAmplifier architecture into an output percentage capacity formatcomprising: a resistance means for a conversion of the input voltagesignal into an input signal current; and a negative feedback means forgenerating a temperature scaled input voltage signal to an instance ofthe given Transconductance Amplifier architecture; such that an outputcurrent of the instance cancels the input signal current while in thepercentage capacity format; and the temperature scaled input voltagesignal is couple to a multiple of Transconductance Amplifiers; and amultiple of biasing currents each having an individual magnitude with acommon absolute temperature scaling being used to current bias amultiple of Transconductance Amplifiers such as to provide both a meansof temperature stabilizing transconductance while also providing a meansof generation for a multiple of voltage signals at a multiple of circuitnodes each having an optimum individual magnitude with the commonabsolute temperature scaling; such that the input voltage signal can becoupled to the multiple of Transconductance Amplifiers in the outputpercentage capacity format; such that an optimum output signalperformance level relative to distortion, gain, transconductance,frequency response, DC offset, temperature drift, and noise can bemaintained over a wide temperature range for the given TransconductanceAmplifier architecture for a multiple of applications.
 3. A method andarchitecture for converting signal from voltage format toTransconductance percentage format of claim 1, wherein means areprovided to convert percentage format into an output voltage format. 4.A method and architecture for converting signal from voltage format toTransconductance percentage format of claim 3, wherein the output of themultiple of Transconductance Amplifiers use in converting frompercentage format to voltage format uses a temperature independentbiased instance of the Transconductance Amplifier architecture having aresistance element coupled to its output.
 5. A method and architecturefor converting signal from voltage format to Transconductance percentageformat of claim 2, wherein the instance of the given TransconductanceAmplifier architecture which cancels the input signal current while inthe percentage capacity format is biased with an input scaledtemperature independent current.
 6. A method and architecture forconverting signal from voltage format to Transconductance percentageformat of claim 1, wherein the output percentage capacity format isdefined to be the differential output current for the givenTransconductance Amplifier architecture expressed as a percentage of amaximum available differential output current.
 7. A method andarchitecture for converting signal from voltage format toTransconductance percentage format of claim 1, wherein an input voltagewhich produces an output current at 50% of the maximum available outputcurrent shall be defined as being an input voltage which is at 50% of apractical maximum input voltage level.